Underground storage for lng



Nov. 11, 1969 H. s. ARENDT UNDERGROUND STORAGE FOR LNG Filed March 15,1968 20 1:11. 2 Y 22 +10 ll '3 *3 *3 '3 v v v"'|"'4 v v L.NG.OUT 4 ms.INJECTION j LNG. INJECTION 4 WELL WELL INITIAL GAS /|NJECT|ON TO a vPURGE WATER ATTOR United States Patent US. Cl. 166252 8 Claims ABSTRACTOF THE DISCLOSURE An underground liquefied gas storage and vaporizationchamber is formed by injecting a purge gas under sufficient pressureinto an aquifier or water bearing sand reservoir to displace the waterand create a gas bubble. Liquefied gas, such as liquefied natural gas(LNG), ethane, ethylene, etc. is injected into the gas bubble whereuponfreezing of the surrounding water in the aquifier occurs. There isappreciable heat flow to the stored liquefied gas from the overlying andunderlying formations which vaporizes a portion of the gas which may bewithdrawn in gaseous form at positions remote from the injection wells.The non-vaporized portion can be withdrawn in liquid form and vaporizedin the usual revaporization facilities.

Background of the invention This invention relates to improvements inmethods of storing and vaporizing liquefied gas in general and inparticular to an improved low cost method of storing and partiallyvaporizing LNG delivered at 2S9 F. and atmospheric pressure or underpressure at more moderate temperatures.

In the transportation of natural gas in liquefied form from a point ofproduction to a point of consumption a major cost factor in the capitalinvestment is the cost of the storage facilities. Conventional types ofatmospheric pressure storage cost in the range of $8.00 per barrel ofcapacity. Recent proposals havesuggested shipments of liquefied naturalgas in the 200. p.s.i. pressure range in order to avoid the extreme lowtemperatures required at atmospheric pressure. The cost of 200 p.s.i.conventional pressure storage soars to approximately $45.00 per barrel.With the large storage volumes required in ship movements of LNG eventhe $8.00 per barrel cost is a major capital investmentland a $45.00 perbarrel cost approaches a prohibitive level. Accordingly, the incentivesfor developing a low cost storage technique for liquefied gases aregreat.

Initial LNG projects were for peak shaving with a relatively smallvolume of gas being liquefied on a daily basis and stored for anextended period for use during winter peaks. Under these conditionsminimum heat flow was critical. Although the new base load LNG projectsin which large volumes of LNG are revaporized each day could not onlytolerate but actually benefit from appreciable heat flow in the storagefacilities. Despite this, the same type of minimum heat flow storagedeveloped for peak shaving projects has been adopted for the new baseload projects.

In accordance with the present invention the cost of providing LNGstorage, either at atmospheric or an elevated pressure such as theaforementioned 200 p.s.i., can be drastically reduced. In accordancewith the invention, the initial step is the selection of an undergroundsane reservoir at a suitable depth and having adequate porosity,permeability and thickness. While a sand reservoir is preferred, asuitable limestone or other aquifier layer can be used. Once a suitableunderground formation has Patented Nov. 11, 1969 ice been selected,injection of gas (such as natural gas, air, flue gas, etc.) equivalentto the desired storage volume or slightly greater to displace the mobilewater out of the storage volume is initiated. The displacement of themobile portion of the formation water is done before LNG is injected toprevent the extreme cold of the LNG from causing an almost immediatefreeze-up and immobilization of the formation water with consequent lossof injectivity. Many aquifiers, i.e. water bearing soil strata, aresufiiciently permeable and extensive so that the displaced water can beforced out into the surrounding aquifier. In other cases, the displacedwater may be produced through wells located around the approximateperiphery of the planned storage area. The use of producing wells toproduce the displaced water will add somewhat to the cost but permitsbetter control of the configuration of the storage bubble as well asminimizing pressure build-up in the aquifier to permit use of lessextensive or less permeable formations. In one form of the novel methodof the invention. LNG injection is initiated as soon as a sufficientlylarge gas slug has been injected so that the gas slug moving ahead ofthe LNG displaces the formation down to about connate water and insuresseparation of the mobile water phase and the LNG. Under this procedureLNG injection is started after a volume of purge gas equal toapproximately /3 to /2 of the ultimate storage volume has beendisplaced.

As the injected LNG ,cools the reservoir, the formation watersurrounding the storage volume freezes, effectively creating animpermeable wall about the storage area and confining the LNG withoutany dependence on structural closure. It is contemplated with thestorage reservoir thus formed that an LNG cargo from an incoming shipmay be injected through suitable injection wells to refill the portionof the reservoir which previous to the ship arrival had been partiallyvoided of liquefied gas by a combination of natural revaporization andwithdrawals in liquid form.

As the stored LNG moves through the reservoir from the injection wellsto the producing wells, a substantial amount of heat is absorbed fromthe formation and a proportionate amount of LNG will be vaporized duringthe precooling of the reservoir. Substantially all of the initial LNGwill be vaporized as the reservoir rock or sand itself is cooled, aswell as the overlying and underlying formations. After this precoolingphase, the major though not exclusive source of heat will be theoverlying and underlying formations which will continue to supplysubstantial heat at gradually diminishing rates as the cooling extendsoutward from the reservoir itself. Although this absorption of heat isvery undesirable in an operation in which LNG is used solely for peakshaving, it is quite acceptable, in fact desirable, where LNG is used ina base load operation. Thus, it is a specific feature and advantage ofthe present invention in providing a system not only for storage of LNGbut in also providing a system for vaporizing a portion of the LNG intogas at the same time. This feature of applicants underground storage(substantial heat flow from the surrounding rock formations) thereforereduces the amount of specific vaporization equipment needed at thepoint of LNG delivery and consumption.

Accordingly, it is the principal object of the invention to provide anew and novel method for storing and vaporizing large quantities ofliquefied gases such as liquefied natural gases.

A further object of the invention is to provide a liquefied gas storagefacility which is extremely safe and non-hazardous to the surroundingenvironment.

A further object of the invention is to provide a low cost storagefacility for LNG which has a substantial heat gain sutficient tovaporize a major portion of the LNG as required.

A further object of the invention is to sufficiently reduce the unitcost of storage so that very large volumes can be stored thus providingsecurity of supply in the event of interruption of deliveries. Thelarger storage will also permit use of larger ships and also the optimumscheduling of ships.

These and other objects and advantages of the invention will becomeapparent and the invention will be fully understood from the followingdescription and drawings in which FIG. 1 is a vertical cross sectionalview of an underground storage reservoir made in accordance with themethod of the invention; and

FIG. 2 is a horizontal cross sectional view of the reservoir of FIG. 1taken along line 2 -2 of FIG. 1.

Referring to the drawings in particular, a better understanding of thenovel method of LNG storage and vaporization system may be had. In FIG.1 a water bearing permeable layer 14 is shown immediately below animpervious rock layer 12 and adjacent an underlying similar imperviousrock layer 16. Layer comprises all of the formations between layer 12and the surface. The aquifer layer 14 while preferably being a waterbearing sand may also be of a porous limestone from which the water maybe similarly purged by a gas under pressure prior to the introduction ofliquefied natural gas. The formation comprising the layers 10, 12, and14 is penetrated by numerous injection and producing wells. A centralinjection well is shown at 18 extending downwardly through the rocklayer 12 and terminating in the upper portion of the aquifer layer 14.After a small bubble of purge gas has been created around injection well18, other injection wells 22 may be added in relatively close proximityto well 18 to increase the purge gas injection rate as well as futureLNG injection capacity. The number of producing and injection wells andtheir relative spacing can be optimized for the conditions of thespecific reservoir and project through well-known reservoir engineeringprinciples. Although a center to periphery pattern is shown forconvenience, this is not the only suitable pattern as individualreservoir conditions or project requirements may suggest the use of anend-toend pattern, a five-spot pattern, line-drive pattern, or any othersuitable arrangement. A plurality of producing wells 20 are arrangedperipherally about the central purge gas injection well 18. All of thewells 18, 20 and 22 are provided with suitable control valves and willbe understood to be connected to suitable pumping means (not shown) asrequired.

In accordance with the novel method of the invention any suitable gas isinjected downwardly under pressure through the well 18 at sufiicientpressure to displace the water from the aquifer layer 14. Obviously, toprevent freezing of the displaced water from the aquifer the displacinggas must be above the freezing temperatures of the formation water. Asthe purge gas is injected through the well 18 an increased hydrostaticpressure is created which tends to cause flow of the water within thearea encompassed by the producing wells 20 outward into the surroundingaquifer. If this aquifer is extensive, all or a major portion of thewater within the planned storage area can be displaced in this manner.However, pressure differentials can be minimized and the configurationof the storage bubble can be controlled by production of water from. theproducing wells 20 during the formation of the storage bubble. When theinjected gas reaches an individual producing well, gas and water will besimultaneously produced, with the ratio of gas to water graduallyincreasing. Prior to gas breakthrough, the relative advance of the gastowards the individual segments of the periphery can be controlled bycontrolling the relative rates of water production from individual wellsor groups of wells. Subsequently, the control can be maintained bycontrolling the relative production rates of individual wells or groupsof wells or by selectively shutting wells 20.

Since the injected gas is less dense than the formation water, gravityeffects will tend to cause this gas to partially override the water inthe formation and, at very slow rates of injection, the gas woulddisplace water from only the top few feet of the formation, leavingwater in the bottom portion. At high injection rates, however, thepressure differentials created will outweigh the gravity effects andwater can be displaced from the entire vertical section, Thisrelationship between vertical displacement and injection rates permitscontrol of the vertical thickness of the bubble and thus the'ratio ofvolume to area. Since the flow of heat into the stored LNG is a directfunctionof area, selection of gas injection rate can roughly optimizethe rate of revaporization.

The injection of purge gas may be continued until the desired storagebubble has been completed. Alternatively, however, after a sufficientpurge gas volume has been injected to create a pre-determined bubblesize indicated by the dotted line 26 (perhaps one-third to one-half theultimate storage volume), injection can be shifted to LNG. The LNG willdisplace the purge gas ahead of it, with the purge gas continuing to actas a buffer between the waterbeing displaced and the extremely cold LNG,and thus avoid the freezing of displaceable water. It is noted that ofthe water will not be displaced as some water will be left behind asresidual water, which will, of course, be frozen as soon as contacted bythe LNG. However, this residual water saturation is low and does notappreciably impair the permeability of the formation. The heat containedin this residual water and the reservoir rock itself will vaporize theinitial LNG injected and this vaporized gas will serve as an additionalbuffer between the LNG and displaceable waters.

Heat will flow indefinitely into the cold sink from the overlying andunderlying formations although at a gradually diminishing rate. As theinjected LNG approaches the periphery of the bubble, the undisplacedwater will be frozen to surround the gas bubble with an impervious icecontainment layer 24 of reservoir rock with pores completely filled withice. In time, this layer will be many hundreds of feet thick.

After the storage bubble is created and the peripheral walls frozen, thebubble is ready for usage. LNG is injected intermittentlyas shipsarrive, with more or less continuing withdrawals as needed. Preferablysufiicient heat will be gained from the surrounding strata(predominately from the overlying and underlying forma tions) so that aportion of the injected LNG will be vaporized and natural gas vapor aswell as LNG will be produced through the producing wells 20 as desired.

The mixture of gas and liquid natural gas can be separated through gasliquid separation facilities. The gaseous portion can be warmed toacceptable distribution temperatures and routed directly to the gasdistribution system. The liquid portion can be sent to conventionalrevaporization facilities and thence to the distribution system. Duringpeak demand periods, the LNG may also be withdrawn from the injectionwells 18 and 22 to increase the amount of LNG available.

Since the revaporized gas will tend to seek the top of the formation andthe heavier LNG the bottom, some control can be exercised over the ratioof gas to liquid that is produced from a given well on any given day byutilizing selective completions. This would involve dual sets ofperforations through the lower end of the well casing (one set near thetop and one near the bottom of the formation) with an appropriate packerand sliding sleeve, dual tubing as other conventional arrangement (notshown).

The ratio of liquid LNG to revaporized gas withdrawn on a given day canalso be controlled by production of selected wells including backflowingof injection wells.

In this manner vaporized LNG may, if desired, be retained in thereservoir so that surface revaporization equipment may be operated atnear capacity.

While specific methods in accordance with the invention have beenillustrated and described in detail to teach the application of theinventive principles, it will be understood that the invention may bepracticed in other ways without depa ing from such principles.

What is claimed is:

1. An improved method of storing liquefied natural gas at cryogenictemperatures comprising the steps of introducing a purge gas into asubterranean water bearing stratum at suificient pressure to displacethe water from a predetermined area of stratur'n and at a temperatureabove the freezing point of the formation water, and thereafterintroducing the liquefied natural gas into the predetermined area tothereby freeze the water surrounding the predetermined area and form an'impermeable barrier to thereby prevent escape of regasified natural gasvapors emerging from said liquefied natural gas.

2. The method of claim 1 wherein said last mentioned step is startedafter a volume of purge gas equal to onethird to one-half of theultimate storage volume has been introduced.

3. The method of claim 1 including the step of producing said displacedwater from at least one well in said stratum at a point remote from thepoint of introduction of said purge gas, and sensing the presence ofpurge gas at the remote water producing well to ascertain and controlthe shape of said predetermined area.

4. The method of claim 3 indluding withdrawing natural gas in a gaseousstate from said stratum after sulficient heat has been absorbed by saidliquefied natural gas from the stratum surrounding said predeterminedarea.

5. A method of storing and vaporizing super-cool liquefied gascomprising the steps of creating a storage void in an aquifier byintroducing a purge gas therein to displace the water from apredetermined area of said aquifer, introducing the super-cool liquefiedgas into said storage void to thereby freezeithe water in the aquifersurrounding said predetermined area, and withdrawing vapors of saidliquefied gas from said predetermined area after sufiicient heat hasbeen absorbed by said liquefied gas from the aquifer surrounding saidpredetermined area.

6. The method of claim 5 wherein the stepfof introducing the super-coolliquefied gas is-commenced after a volume of purge gas equal toone-third to one-half of the ultimate storage volume of the storage voidhas been produced.

7. The method of storing and vaporizing liquefied natural gas (LNG)delivered in 'ifa ship to a consuming area at 259 F. and atmosphericpressure comprising the steps of, drilling a first injection wellthrough a relatively impervious rock layer into water bearing sandstratum immediately therebelow, drilling a plurality of producing wellsthrough said rock stratum into said sand stratum to approximately thesame'gg jepth as said injection well, said producing wells being drilledin a substantially circular pattern concentric to and spaced from saidinjection well, drilling one or more LNG injection wells in closeproximity to said first injecition well, injecting a purge gas at atemperature abov *32" F. through said first injection well to displaceth ater from the sand stratum and at the same time produfiing water fromsaid producing wells to thereby create la predetermined area in saidsand stratum free of waterffclosing said producing wells upon change ofproduction from water to purge gas, introducing liquefied natural gasthrough said LNG injection wells to thereby freeze the water in saidstratum surrounding said predetermined' area, and withdrawing gasifiednatural gas from said plurality of producing wells.

8. The method of claim wherein the step of injecting purge gas is doneat a ratesufficiently slow to create a large diameter predetermined areaof minimum depth free of water immediately below, said impervious rocklayer, whereby maximum dissipatin of LNG cold will occur in the radialmovement of LNG outward from the LNG injection wells to the gasiproducing wells.

References Cited UNITED STATES PATENTS 3,275,078 9/1966 Rieljer 166-3053,296,805 1/1967 Graham 166-305 3,301,326 1/1967 McNamer 166-285 X3,304,725 2/1967 Faul coner 61--.5 3,306,354 2/1967 OBfiien 166-3053,344,607 10/1967 Vigjovich 61.5 3,393,738 7/1968 Ber 'ard et a1. l66305X OTHER REFERENCES STEPHEN J. NOVOSAD, Primary Examiner US. Cl. X.R.

